a. Field of the Invention
The invention relates to electrolytes and organic solvents for electrochemical cells. In particular, the invention relates to lithium ion electrolytes and organic solvents for lithium ion cells.
b. Background Art
Lithium ion cells typically include a carbon (e.g., coke or graphite) anode intercalated with lithium ions to form LixC; an electrolyte consisting of a lithium salt dissolved in one or more organic solvents; and a cathode made of an electrochemically active material, typically an insertion compound, such as LiCoO2. During cell discharge, lithium ions pass from the carbon anode, through the electrolyte to the cathode, where the ions are taken up with the simultaneous release of electrical energy. During cell recharge, lithium ions are transferred back to the anode, where they reintercalate into the carbon matrix.
Lithium ion rechargeable batteries have the demonstrated characteristics of high energy density, high voltage, and excellent cycle life. Known state-of-the-art lithium ion rechargeable batteries and systems have been demonstrated to operate over a wide range of temperatures (e.g., −30° C. (Celsius) to +40° C.). However, the performance of such known lithium ion rechargeable batteries and systems is limited at temperatures below −30° C., making them unsuitable for many terrestrial and extra-terrestrial applications. Many scheduled NASA missions demand good low temperature battery performance without sacrificing such properties as light weight, high specific energy, long cycle life, and moderate cost. Moreover, such scheduled NASA missions require rechargeable batteries that can operate at low temperatures to satisfy the requirements of various applications, such as landers, rovers, and penetrators. For example, the Mars Exploration Program requires rechargeable batteries capable of delivering several hundred cycles with high specific energy, and the ability to operate over a broad range of temperatures, including the extremely low temperatures on and beneath the surface of Mars. Mars rovers and landers require batteries that can operate at temperatures as low as −40° C. Mars penetrators, which can penetrate deep into the Martian surface, require operation at temperatures less than −60° C. Additional applications may require high specific energy batteries that can operate at temperatures down to −80° C., while still providing adequate performance and stability at ambient temperatures.
To be used on the Mars missions and in low earth orbit (LEO) and geostationary earth orbit (GEO) satellites, as well as in terrestrial applications, lithium ion rechargeable batteries may exhibit high specific energy (60-80 Wh/Kg (Watt hours per Kilogram)) and long cycle life (e.g., >500 cycles).
Known state-of-the-art lithium ion cells typically exhibit limited capacities below −30° C. This may be due to limitations of the electrolyte solutions, which become very viscous and freeze at low temperatures, resulting in poor electrolyte conductivity. In addition, the surface film, such as solid electrolyte interphase (SEI), that forms on the electrodes, can either build up over the course of repeated charge/discharge cycling or become highly resistive at lower temperatures. Ideally, the SEI layer on the carbon anode should be protective toward electrolyte reduction and yet conductive to lithium ions to facilitate lithium ion intercalation, even at low temperatures.
Several factors can influence the low temperature performance of lithium ion cells, including: (a) the physical properties of the electrolyte, such as conductivity (lithium ion mobility in the electrolyte solution), melting point, viscosity, and other physical properties; (b) the electrode type; (c) the nature of the SEI layers that can form on the electrode surfaces; (d) the cell design; and, (e) the electrode thickness, separator porosity and separator wetting properties. Of these factors, the physical properties of the electrolyte typically have the predominant impact upon low temperature performance, as sufficient electrolyte conductivity is typically a condition for good performance at low temperatures. Ideally, a good low temperature performance electrolyte solvent should have a combination of properties such as high dielectric constant, low viscosity, adequate Lewis acid-base coordination behavior, as well as appropriate liquid ranges and salt solubilities in the medium.
Known electrolytes used in state-of-the-art lithium ion cells have typically consisted of binary mixtures of organic solvents, for example, high proportions of ethylene carbonate, propylene carbonate or dimethyl carbonate, within which is dispersed a lithium salt, such as lithium hexafluorophosphate (LiPF6). Examples may include 1.0 M (Molar) LiPF6 in a 50:50 mixture of ethylene carbonate/dimethyl carbonate, or ethylene carbonate/diethyl carbonate. Such electrolytes typically do not perform well at low temperatures because they become highly viscous and/or freeze.
Optimized electrolyte formulations consisting of a ternary, equi-proportion mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) were disclosed in U.S. Pat. No. 6,492,064 to Smart et al. In addition, lithium ion cells with a quaternary electrolyte formulation consisting of 1.0 M LiPF6 EC+DEC+DMC+EMC (1:1:1:2 v/v), as well as low EC (ethylene carbonate)-content quaternary solvent blend electrolytes, which have enabled excellent performance down to −50° C., are known. However, such ternary and quaternary electrolyte formulations may not provide good cell rate capability at temperatures below −50° C., primarily due to poor ionic conductivity.
Improved performance with multi-component electrolytes of the following formulation: 1.0 M LiPF6 in ethylene carbonate (EC)+ethyl methyl carbonate (EMC)+X (1:1:8 v/v %) (where X is methyl butyrate (MB), ethyl butyrate (EB), methyl propionate (MP), and ethyl valerate (EV)) are also known. Although such electrolyte formulations do provide good performance at very low temperatures, the high temperature resilience of cells containing such electrolytes may be compromised, primarily due to the use of small quantities of ethylene carbonate and high quantities of the ester component.
The use of methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), ethyl propionate (EP), and ethyl butyrate (EB) in multi-component electrolyte formulations is known. However, although some of these electrolytes provide good low temperature performance, they generally do not result in cells with good rate capability at lower temperatures and do not display good high temperature resilience (e.g., >25° C.).
Higher molecular weight esters, such as ethyl propionate and ethyl butyrate, resulting in both improved low temperature performance and good stability at ambient temperatures, were disclosed in M. C. Smart, B. V. Ratnakumar, S. Surampudi, J. Electrochem. Soc., 149 (4), A361, (2002), where excellent performance was obtained down to −40° C. with electrolytes consisting of the following formulations: a) 1.0 M LiPF6 EC+DEC+DMC+ethyl butyrate (EB) (1:1:1:1 v/v %) and b) 1.0 M LiPF6 EC+DEC+DMC+ethyl propionate (EP) (1:1:1:1 v/v %). However, although electrolytes containing methyl acetate and ethyl acetate (low molecular esters) were shown to result in high conductivity at low temperatures and good cell performance at low temperature initially, their high reactivity toward the anode led to continued cell degradation and poor long term performance.
In addition, ester based co solvents having improved low temperature performance were disclosed in A. Ohta, H. Koshina, H. Okuno, and H. Murai, J. Power Sources, 54 (1), 6-10, (1995), where electrolytes consisting of the following formulations were disclosed: a) 1.5 M LiPF6 in EC+DEC+MA (1:2:2), b) 1.5 M LiPF6 in EC+DEC+MP (1:2:2), and c) 1.5 M LiPF6 in EC+DEC+EP (1:2:2). However, the incorporation of a large proportion of diethyl carbonate (DEC) produced undesirable effects upon the surface films of carbon anodes.
In addition, electrolytes containing ethyl acetate (EA) and methyl butyrate (MB) were disclosed in S. Herreyre, O. Huchet, S. Barusseau, F. Perton, J. M. Bodet, and Ph. Biensan, J. Power Sources, 97-98, 576 (2001) and in U.S. Pat. No. 6,399,255 to Herreyre et al., where electrolytes consisting of the following formulations were disclosed: a) 1.0 M LiPF6 in EC+DMC+MA, b) 1.0 M LiPF6 in EC+DMC+MB, c) 1.0 M LiPF6 in EC+PC+MB, and d) 1.0 M LiPF6 in EC+DMC+EA (solvent ratios not provided). Good low temperature performance with the methyl butyrate-based electrolyte was disclosed. However, the performance at temperatures below −40° C. was not investigated.
In addition, electrolytes containing methyl acetate and ethyl acetate in ternary mixtures with and without blending with toluene were disclosed in H. C. Shiao, D. Chua, H. P., Lin, S. Slane, and M. Solomon, J. Power Sources, 87, 167-173 (2000), in an attempt to obtain improved performance at temperatures as low as −50° C. However, such improved performance at temperatures as low as −50° C. was not shown.
In addition, the performance of electrolyte formulations at low temperatures were disclosed in S. V. Sazhin, M. Y. Khimchenko, Y. N. Tritenichenko, and H. S. Lim, J. Power Sources, 87, 112-117 (2000), where electrolytes consisting of the following formulations were disclosed: a) 1.0 M LiPF6 in EC+EMC+EA (30:30:40), b) 1.0 M LiPF6 in EC+DMC+MA (30:35:35), c) 1.0 M LiPF6 in EC+DEC+EP (30:35:35), and d) 1.0 M LiPF6 in EC+EMC+EP (30:30:40). Although good performance of the electrolytes was demonstrated at −20° C., the performance at temperatures below −20° C. was not investigated. However, at very low temperatures (<−40° C.) the high EC (ethylene carbonate)-content (30%) and low proportion of the ester-based component (30-40%) in these formulations was not anticipated to yield good performance.
Accordingly, there is a need for lithium ion electrolytes for use with lithium ion cells with improved low temperature performance over known electrolytes and cells.